Methane is associated in varying amounts with most coal deposits. It may be formed thermogenically during burial and maturation of the coal or it may be produced biogenically by the action of microbes. Bacteria are considered to be the primary degraders of coal, producing a range of intermediates which are successively degraded to methane precursors such as hydrogen gas, carbon dioxide, acetate and various others compounds (e.g. dimethyl sulfide, formate, methanol and methylamines). These precursors are then converted to methane via methanogenic archaea. This methanogenic process may occur via a number of mechanisms including CO2 reduction, acetoclastic (from acetate) or methylotrophic processes.
The coal seam environment in which biogenic methane is produced is anoxic and reducing. Due to macronutrient limitation biogenic methane production is slow and occurs over long time-scales. Production from a typical coal seam methane (CSM) well may occur for 5-7 years, after which time the rate of production becomes uneconomic and the well may be abandoned.
It may be possible to prolong the production life of the well by introducing methanogenic microbial populations. US Publication No. 2004/0033557 describes introducing a consortium of selected anaerobic microorganisms into a subsurface formation for in situ conversion of organic compounds in the formation into methane and other compounds.
It may also be possible to relatively rapidly replenish the methane within a buried coal seam by stimulation of the microbes that reside in the coal and/or associated water. It is known that this can be achieved by addition of nutrients to the system. For example, U.S. Pat. No. 7,832,475 describes a method for enhancement of biogenic methane production that involves introducing an indiscriminate microbial population stimulation amendment, such as corn syrup, emulsified oil, and milk, to blanket boost microbial populations in a hydrocarbon-bearing formation. The method further involves subsequent manipulation of the microbial populations by selectively starving one or more microbial populations to selectively sustain at least one of the boosted microbial populations.
In processes designed to enhance methane production, through enhancing microbial activity through nutritional supplements, the lifecycle analysis of the whole system needs to be taken into account. For example, U.S. Pat. No. 4,826,769 discloses that microbial produced methane from coal preferably requires a ratio of C:N:P of 100:5:1. With nitrogen typically derived via energy intensive sources such as ammonia production, efficient and effective dosage regimes are needed.
Whilst significant progress has been made in increasing methane production through enhancing growth in consortia of microbes, there is still further scope for improvement.